1. Field of the Invention
This invention relates to a method of and apparatus for stabilizing images in electrophotography. More particularly, it relates to a method of and apparatus for measuring the surface potential of light and dark regions of an electrostatic image and detecting the difference or contrast between said surface potentials to thereby stabilize the image.
2. Description of the Prior Art
It is well known that electrophotography is a method whereby charge and a light image are applied to a photosensitive medium to form thereon an electrostatic latent image corresponding to the light image and imparting powder or liquid developer to such a latent image to visualize the latent image. This method has been widely put into practice as office copiers. However, the image formed by the machine may be unstable due to various factors and are not sufficiently satisfactory to the users. As typical factors for the unstability of the image, mention can be made of the deterioration of the photosensitive medium and fluctuations of charging, usually of corona charger due to temperature and humidity. These fluctuations can be detected by measuring the surface potentials in the step of electrostatic image formation which is an intermediate step of the image formation process, and it is known to change the various conditions for the electrostatic image formation in accordance with the detection signal to thereby form a stable electrostatic image.
For example, U.S. Pat. No. 2,956,487 discloses a method in which a probe is disposed in opposed relationship with latent image to thereby effect sensing signal which is a function of the color value of the latent electrostatic image area to thereby stabilize the developed image. U.S. Pat. No. 3,604,925 discloses an apparatus for automatically controlling the amount of electrostatic charge applied to a plane by controlling the potential applied to a corona wire. U.S. Pat. No. 3,788,739 discloses measurement of the surface potential by a probe which is positioned only in the position of the area which is always light struck, and comparison of the surface potential to a fixed reference in order to provide a control signal for compensation in an electrophotographic reproduction device. Further, U.S. Pat. No. 4,000,944 discloses a built-in electrode at the end of a photoreceptor and a probe disposed in opposed relationship therewith to thereby detect the amount of charging effected by a charger and to control so that the charge may be a desired charge on the drum photoconductive surface.
None of such prior art techniques is directed to the detection of the surface potentials of light and dark regions of an electrostatic image and detection of the difference between these surface potentials to thereby directly detect the contrast of the electrostatic image.
In our copending U.S. Patent Application Ser. No. 832,944 (German Patent Application No. P27 41 713.6; British Patent Application No. 38136/77; French Patent Application No. 77/28016), there is disclosed a method of stabilizing an electrostatic latent image which includes the steps of measuring the dark region potential and the light region potential on the photosensitive medium, comparing the thus measured values with respective predetermined reference potential of the light and dark regions, and when the differences thereof are not within the allowable predetermined ranges, then controlling the necessary factors such as energy source of chargers.